Process for the preparation of an acrylonitrile copolymer and product prepared therefrom

ABSTRACT

A process for the rapid precipitation polymerization of acrylonitrile and a minor amount of a vinyl carboxylic acid comonomer in an environment of less than 10 ppm metal-ions to produce an acrylonitrile copolymer which, when pyrolyzed in an oxidizing atmosphere, produces a high quality carbon fiber.

This is a divisional of application Ser. No. 08/057,470 filed on May 6,1993, now U.S. Pat. No. 5,364,581.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing anacrylonitrile copolymer which is a precursor in the production of highquality carbon fibers. More specifically the invention relates to aprocess for producing acrylonitrile copolymer in an environment of lessthan 10 ppm of metal ions. The copolymer so produced is readily spuninto fibers and then thermally converted into carbon fibers.

Carbon fibers prepared from acrylonitrile polymers and copolymers by arapid oxidation process have superior physical properties such asincreased tensile strength. The fibers are useful as reinforcementmaterials in automobile, aerospace, recreational and various otherindustries. An increasing demand for strong, lightweight materialsinsures an expanded use of carbon fibers in the future. Thus a needexists for a process which insures that the starting materials forproducing carbon fibers are of the finest quality. A fine qualityacrylonitrile polymer or copolymer has no defects such as voids formedwhen gases are expelled during fiber preparation. Also the fiber shouldnot contain more than traces of metal contaminants, as these tend todegrade the fiber. The fiber should have a round shape for maximumstiffness.

Carbon fibers, which have heretofore been used as reinforcing materialfor plastic composite compositions, are preferably characterized by hightensile strength, high rigidity and a homogeneous fibrous structure.These characteristics can be adversely affected by certain propertiesfound in the acrylonitrile copolymer feedstocks. If these undesirableproperties can be identified and removed, then the final carbon fiberproduct is greatly enhanced in desirable characteristics.

U.S. Pat. No. 4,062,857 (Kobashi et al) discloses a process forpreparing an acrylonitrile polymer in a substantially melted state. Thepolymerization, which is conducted under pressure, produces a polymer ofuniform molecular weight with very little by-product. The patent doesnot exclude the use of vinyl sulfonic acid as a comonomer (Col. 2, line65).

U.S. Pat. No. 4,049,605 (Kobashi et al) discloses a process forpreparing an acrylonitrile polymer in a substantially molten state. Thepolymerization is conducted under increased pressure and in the presenceof a catalyst such as a peroxide. The patent does not exclude the use ofvinyl sulfonic acid or salts thereof as a comonomer (Col. 3, line 26).

U.S. Pat. No. 3,917,776 (Sato et al) discloses a process for producing ahigh quality carbon fiber. The feedstock can comprise an acrylonitrilecopolymer containing units of acrylic acid, methacrylic acid or itaconicacid. Substantial amounts of a metal-containing catalyst are employed(claim 1).

The above-listed patents are incorporated herein by reference.

The process disclosed herein provides for the formation of acrylonitrilecopolymer fibers having highest quality properties such as freedom fromharmful sulfonic acid residues, freedom from more than 10 ppm metal ioncontaminants, freedom from voids, freedom from strong acids, suitablyhigh molecular weight and increased stiffness.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor rapidly and economically producing a high quality acrylonitrilecopolymer which can be directly provided to a spinning operation andthen to pyrolysis to obtain an improved carbon fiber product.

It is another object of the present invention to provide a productcomprising an acrylonitrile copolymer which is substantially free ofmetal ions and sulfonic acid groups.

It is a further object of the present invention to provide an improvedcarbon fiber product prepared from the metal ion-free, sulfonic acidgroup-free acrylonitrile copolymer.

These and other objects have now herein been attained by a precipitationpolymerization process comprising maintaining in a reaction zone asolvent system comprising water and an organic solvent capable ofdissolving acrylonitrile copolymer, but in ratios such that the polymeris in the form of a slurry. A vinyl sulfonic acid comonomer-freefeedstock is then added to the reaction zone. The feedstock comprises amajor amount of acrylonitrile monomer and minor amount of a vinylcarboxylic acid monomer. Then an initiator system comprising a peroxideand an organic mercaptan and 2 ppm of an iron catalyst is added to thereaction zone. The mixture of solvent system, comonomers and initiatorsis then subjected to suitable conditions of temperature and pressure toobtain a suspension of acrylonitrile copolymer. The copolymer hasimproved physicals which results, upon pyrolysis, in a high qualitycarbon fiber product.

After polymerization is complete, water and excess monomer are removedas by stripping and then appropriate amounts of solvent are added togive a spinnable solution of acrylonitrile copolymer. The solution issubjected to wet spinning to provide round fibers. There is no need topurify the polymer solution prior to spinning.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention provides for acrylonitrilecopolymer fibers which are excellent starting materials for preparingcarbon fibers. The process includes a very rapid precipitationpolymerization which gives a product having a good molecular weight. Ina preferred embodiment, the copolymer prepared from precipitationpolymerization has a molecular weight of about 40,000 to about 100,000no. average molecular weight. The polymerization is allowed to continueuntil the polymer mix contains 20% to 40% solids. Preferably, theintrinsic viscosity of the final polymer is about 1.1 to 1.7.

By precipitation polymerization is meant a polymerization processwherein the growing polymer comes out of solution at a certain stage,usually when about ten monomer units have been polymerized to form apolymer chain. Once out of solution, the polymer is unaffected byinitiators and the like which tend to chain-stop the polymer. Monomer isable to penetrate the polymer and allows for the rapid continued growthof the polymer chain to a high molecular weight. Since the polymergrowth is rapid, precipitation polymerization can be conducted in acontinuous manner.

The solvent system used in the precipitation polymerization preferablycomprises a mixture of water and at least one other organic solvent. Theorganic solvent or solvents must be capable of dissolving acrylonitrilecopolymer of a molecular weight as described above. In a preferredembodiment, the organic solvent is a member selected from the groupconsisting of dimethylformamide, dimethyl acetamide, dimethyl sulfoxide,tetramethylene cyclic sulfone and butyrolactone. The organic solvent orsolvents can be present in the aqueous solvent system in an amount offrom about 30% by volume to about 90% by volume. It has been found thatthe polymerization rate of the comonomers is dramatically increased whenthe water plus the solvent does not completely dissolve the copolymer.In a preferred embodiment, a solvent system comprising dimethylformamide(DMF) and water is employed, but dimethyl sulfoxide and water are alsogood. It has been observed that the itaconic acid mixture preventsformation of voids when neutralized and the fibers are formed from wetspinning. Precipitation polymerization is at least ten times faster thansolution polymerization.

The organic solvent which is selected for mixture with water to form asolvent system for solution polymerization must be free from metals andmetal ions. Examples of solvents not suitable for the practice of thepresent invention are sodium thiocyanate and zinc chloride.

The feedstock for the precipitation polymerization comprises a majoramount of acrylonitrile monomer and a minor amount of a vinyl carboxylicacid comonomer. In a preferred embodiment, the acrylonitrile monomer ispresent in the feedstock in an amount from about 85% by weight to about99% by weight. In a most preferred embodiment, the acrylonitrile monomeris present in an amount from about 92% by weight to about 98% by weight.

The vinyl carboxylic acid comonomer is a member selected from the groupconsisting of itaconic acid, acrylic acid and methacrylic acid. It iswithin the scope of the present process to use more than one comonomer.In addition to carboxylic acid-containing comonomers, other olefinicmonomers can also be present. The only restriction imposed on thepresent process is that a vinyl sulfonic acid comonomer, allyl sulfonicacid comonomer, salts thereof, and the like cannot be included in thefeedstock compositions. It has been observed that the presence ofsulfonic acid groups in the final acrylonitrile copolymer causesretention of metal ions. The feedstock for use in the present processmust be substantially free of sulfonic acid groups. By substantiallyfree of sulfonic acid groups is meant not more than 0.5 mole % sulfonicacid groups based on the polymer composition. Also, when sulfonic acidgroups are replaced by carboxyl groups in the final acrylonitrilecopolymer, the oxidation rate during carbon fiber preparation isincreased.

The precipitation polymerization of acrylonitrile monomer and suitablecomonomers is enhanced-by the presence of an initiator system comprisinga mixture of a peroxide and a low molecular weight volatile organicmercaptan. The organic mercaptan should have low volatility. Theinitiator system cannot contain any metal or metal ion-containingcompounds greater than 10 ppm. The peroxide is a member selected fromthe group Consisting of hydrogen peroxide, t-butyl hydroperoxide,t-butyl peroxide and lauroyl peroxide. The mercaptan is a memberselected from the group consisting of 1-thioglycerol, mercapto ethanoland butyl mercaptan isomers. By butyl mercaptan isomers is meant normalbutyl mercaptan, sec-butyl mercaptan and iso-butyl mercaptan. By lowmolecular weight organic mercaptan is meant a C₁ -C₆ organic mercaptan.

A catalytic amount of an iron compound is added to the mixture ofsolvent, initiator and monomer systems. Examples of iron catalysts areferric (or ferrous) nitrate, ferric (or ferrous) chloride, and ferric(or ferrous) ammonium sulfate. The compounds can have water of hydrationassociated therewith.

An acrylonitrile copolymer is prepared by reacting the mixture ofmonomers, solvents and initiators under suitable conditions oftemperature and pressure. In a preferred embodiment, the reaction isconducted at a temperature of about 50° C. to about 70° C.; and at apressure of about 1.0 atmospheres to about 1.2 atmospheres. As thepolymerization continues, feedstock, solvent and initiator can be addedeither in a continuous fashion or at regular intervals to maintaincorrect amounts of reactants and the like in accordance with parameterswell-known to those skilled in the art. As was noted earlier, preferablythe polymerization is continued until the solids content reaches about20% to about 40%. The precipitation polymerization provides for a rapidrate of conversion and a high molecular weight product.

The present process is best used in a continuous precipitationpolymerization which allows good dissipation of the heat ofpolymerization and allows reaction times as short as 30-60 minutes.Current solution polymerization requires 17-24 hours; attempts to speedsolution polymerizations result in unusually low molecular weightpolymer.

Once the polymerization is completed, the water and unreactedacrylonitrile are removed as by stripping, and the acrylonitrilecopolymer dissolves in the organic solvent. Additional organic solventis added to adjust solids to the proper viscosity. The usual steps ofisolating the copolymer, washing, drying, grinding, and dissolving priorto spinning are not performed. Rather the dissolved copolymer isdirectly converted into fibers by wet spinning to obtain high qualityround fibers suitable for preparing carbon fibers as by oxidativepyrolysis.

In a preferred embodiment, precipitation polymerization is conductedcontinuously in a vertical reactor column. Monomer is added at the topof the column and polymer is withdrawn from the bottom of the column. Atthe top of the column a cloudy suspension of polymer is observed, and atthe bottom of the column a clear solution of polymer is observed. Aswater and monomer are continuously drawn off, the polymer becomesincreasingly soluble in the solvent mixture as the charge advancestoward the bottom of the vertical reactor column.

Another feature of the present process is the addition of a specifictype of oxidation catalyst to the acrylonitrile copolymer prior tocarbonization. The oxidation catalyst must be free of metal or metalions. The oxidation catalyst is a member selected from the groupconsisting of ammonia and low molecular weight amines. By low molecularweight amines is meant a C₁ -C₆ aliphatic amine. The oxidation catalystcan be added to the acrylonitrile copolymer either before the wetspinning operation or after wet spinning.

The process for converting oxidation catalyst-containing acrylonitrilecopolymer fibers into high quality carbon fibers can be selected fromany one of the well-known processes for carbonization. The carbonizationprocess as such forms no part of the present invention. The presence ofthe ammonia or amine catalyst allows for an increase in fiber density toabove 1.30 in the oxidation process which is completed in a time ofabout six minutes. Now it is ready for the carbonization step. Voidformation in the fibers is prevented by the use of amine catalysts.Other advantages of using the amine catalysts are improved heatdissipation and absence of particulate residues. Carbon fibers obtainedfrom acrylonitrile copolymer fibers prepared according to the presentprocess have excellent properties.

The present invention will be explained in further detail by way ofexample, but the scope of the invention is not limited by the example.In the example, parts and percentages are by weight unless otherwisespecified.

EXAMPLE

A jacketed aluminum reactor is charged with a solvent mixture ofdimethylformamide and water in a ratio of about 80:20 by volume and inan amount of 0.6 liters. An acrylonitrile monomer and an itaconic acidmonomer is added to the reactor in a ratio of about 95:5 by weight andin an amount of 400 grams. Hydrogen peroxide in an amount of about 0.5%based on total monomer weight is then added to the reactor.1-Thioglycerol is added to the reactor in an amount of about 0.3% basedon total monomer weight. Ferrous Ammonium Sulfate Hydrate is added tothe reactor in an amount of about 10 ppm based on total reagents.

The slurry or precipitation polymerization is conducted in the aluminumreactor at a temperature of about 50° C. and a pressure of about 1.0atmospheres. Polymerization is continued until the solids contentreaches about 25% to 30% solids (about 1 hour). After polymerization iscompleted, a vacuum stripping system is employed to remove water andunreacted monomer. The stripping operation yields a direct spin dopewithout isolation of polymer. To reduce the solids to 30%, add 220 gramsof DMF. Ammonia is then added to the spin dope in an amount of 6 gramswith thorough mixing. The solution of polymer and oxidation catalyst(ammonia) is then subjected to a wet spinning operation to obtaincatalyst-containing acrylonitrile copolymer fibers.

Many equivalent modifications will be apparent from a reading of theabove to those skilled in the art without a departure from the inventiveconcept which is limited and defined only by the appended claims.

We claim:
 1. A process for the precipitation polymerization ofacrylonitrile comprising:maintaining in a reaction zone a substantiallymetal-ion free solvent mixture comprising water and an organic solventcapable of dissolving acrylonitrile copolymer, adding to the reactionzone a vinyl sulfonic acid comonomer-free feedstock comprising a majoramount of acrylonitrile monomer and a minor amount of a vinyl carboxylicacid comonomer, adding to the reaction zone an initiator mixturecomprising a peroxide, a low molecular weight organic mercaptan, andless than 10 ppm of metal ion, and polymerizing the acrylonitrilemonomer and vinyl carboxylic acid monomer under suitable conditions oftemperature and pressure to obtain an acrylonitrile copolymer.
 2. Aprocess according to claim 1 wherein the organic solvent capable ofdissolving acrylonitrile copolymer is a member selected from the groupconsisting of dimethylformamide, dimethyl acetamide, dimethyl sulfoxide,tetramethylene cyclic sulfone and butyrolactone.
 3. A process accordingto claim 1 wherein the vinyl carboxylic acid comonomer is a memberselected from the group consisting of itaconic acid, acrylic acid andmethacrylic acid.
 4. A process according to claim 1 wherein the peroxideis a member selected from the group consisting of hydrogen peroxide,t-butyl hydroperoxide, t-butyl peroxide and lauroyl peroxide.
 5. Aprocess according to claim 1 wherein the low molecular weight organicmercaptan is a member selected from the group consisting of1-thioglycerol, mercapto ethanol and butyl mercaptan isomers.
 6. Anacrylonitrile copolymer prepared according to the process of claim 1.